Portland police arrested an 11-year-old boy Thursday night after he allegedly threatening a woman on a TriMet MAX train after bumping into her baby's stroller.

A group of teenagers boarded the train at the Skidmore Fountain station and bumped into the stroller about 8:20 p.m., said Sgt. Pete Simpson, a spokesman for the Portland Police Bureau. The woman said something to the group, which sparked an argument.

The 11-year-old then walked up to the woman, Simpson said.

"The kid lifts up his shirt and there's a gun," he said.

He started the pull the gun out of the waistband of his pants and the woman backed off and called 9-1-1. She pointed out the suspect to officers when the train stopped at the Old Town/Chinatown platform, Simpson said.

Officers took the 11-year-old into custody after finding a loaded .22 caliber semi-automatic handgun in his waistband. They also arrested a 13-year-old boy who had a BB gun in his backpack.

"I'm blown away," Simpson said. "I'm a cop for 18 years and I've never heard of an 11-year-old with a gun."

The 13-year-old was taken to Janus Youth Program. Officers obtained a court order to lodge the 11-year-old in the Donald E. Long Juvenile Detention Home.